Variability in maturity and growth in a heavily exploited stock: whiting (Merlangius merlangus L.) in the Irish Sea

This paper examines the relationships between maturity, length and age of whiting sampled on a length-stratified basis from groundfish surveys of the Irish Sea during spawning in spring 1992–2001. Maturity, defined by the triggering of vitellogenesis or milt production, was a function of both length and age. Proportions of mature individuals in 1-year-old males increased successively from almost zero in length classes below 15 cm to around 0.9 at 25 cm, whilst almost all 2-year-old males were mature from their smallest length of around 19 cm. Maturity in females was more strongly linked to age than to length. Most 1-year-old females were immature, the proportion of mature individuals not exceeding 0.3 in any length class. Most 2-year-old females were mature and immature fish were found in the smallest length classes only (20–25 cm). Almost all 3-year-olds of both sexes were mature in all length classes. Proportions of mature individuals in 1-year-olds increased substantially after 1997, particularly in males. Significant positive cross-correlation between proportion mature and mean length was found for 1-year-olds of both sexes. Length at 50% maturity (L50) averaged around 19 cm in males and 22 cm in females. Variability in L50 was negatively cross-correlated with average sea surface temperature in the preceding year. There is no evidence for substantial changes in maturity of whiting since the 1950s, despite an order-of-magnitude reduction in biomass caused by high fishing mortality. Concomitant decreases in mean length-at-age and weight-at-age in recent decades indicate that conditions may have been unfavourable for compensatory changes in maturation.